Oscillators operative to generate clock signals are inherently inaccurate due to manufacturing limitations and sensitivity to temperature variations. Oscillators inaccuracy is typically measured in parts per million (PPM). An oscillator's accuracy measures the possible range of error between the frequency generated by the oscillator and the manufacturer listed frequency, typically expressed in PPM. According to one example, an oscillator has a manufacturer listed frequency of 1 MHz and an accuracy of 10 PPM. The actual frequency generated by the oscillator may range from 999,990 Hz to 1,000,010 Hz. According to another example, an oscillator produces a clock frequency of 1,000,005 Hz at 10 degrees Celsius and 1,000,010 at 20 degrees Celsius. For this and other reasons, two oscillators located in a transmitter and a receiver of a communication system are unlikely to generate the same frequency even if their manufacturer listed frequencies are identical.
Two methods deployed for clock distribution in packet networks are synchronous-Ethernet (ITU-T G.8262) and IEEE 1588V.2. The synchronous-Ethernet method can distribute clock frequency information through locking of the Ethernet clock, but cannot distribute phase and TOD information. The IEEE 1588V.2 can distribute any type of clock information, but is much more sensitive to issues such as packet delay variation, packet loss and network behavior.